Cathodic protection system



Oct. 28, 1969 H. w. HosFoR D 3,475,311

CATHODIC PROTECTION SYSTEM Filed Nov. 22, 1965 I ns m is 1+ TANKINVENTOR HARRY W HOSFUE BY MM ($0 ATTORNEY.

United States Patent 3,475,311 CATHODIC PROTECTION SYSTEM Harry W.Hosford, Shaker Heights, Ohio, assignor to gligco Corporation,Cleveland, Ohio, a corporation of Filed Nov. 22, 1965, Ser. No. 509,067Int. Cl. C23f 13/00 U.S. Cl. 204-196 4 Claims ABSTRACT OF THE DISCLOSUREThe present invention relates generally, as indicated, to a protectivesystem and, more particularly, to certain improvements inself-regulating cathodic protection systems especially of the type shownand described in U.S. Letters Patent No. 3,143,670, granted to BernardHusock on Aug. 4, 1964.

In such prior patent, there is disclosed a relatively simple andinexpensive impressed current cathodic pro tection system which isadapted to produce a substantially constant current at any given anodedespite fluctuations in the load resistance between the anode and astructure to be protected, such as a gasoline tank, by placing a fixedresistor having a resistance at least ten times as great as theresistance between the anode and protected structure in series with arectifier for converting a typical 115 volt A.C. source to directcurrent. Because the resistance of the fixed resistor is quite high ascompared to the maximum change in resistance that might occur in theelectrolyte between the anode and protected structure from wet to dryperiods or for other reasons, it has been found that the current outputwill always deviate less than percent.

Thus, by employing the protective system of the aforementioned Husockpatent, it is possible to provide the minimum current flow which willprotect the buried structure, since there is no danger of a substantialdrop in such flow as operating conditions change. However, in someinstances it is desirable to eliminate most of the heat which isgenerated by the resistors for longer component life and still providethe constant current output at the anodes despite fluctuations in theload resistance between the anodes and structure being protected, whichis a principal object of this invention.

Another object is to provide an improved cathodic protection system withnovel means for regulating the voltage output at the anodes.

Still another object is to provide a cathodic protection system of thetype indicated with means for regulating the current output at theanodes.

A further object is to provide such a protective system with novel meansfor automatically shutting the system oli when there is an interruptionin the flow of current, thus eliminating any possibility of sparkingaround the protected structure should a break occur in the circuit.

Other objects and advantages of the present invention will becomeapparent as the following description proceeds.

To the accomplishment of the foregoing and related ends, the invention,then, comprises the features hereinafter fully described andparticularly pointed out in the claims, the following description andthe annexed drawing setting forth in detail a certain illustrativeembodiment of the invention, this being indicative, however, of but oneof the several ways in which the principles of the invention may beemployed.

In such annexed drawing there is shown a schematic circuit diagram of apreferred form of protective system in accordance with the presentinvention.

The protective system disclosed herein by Way of example is generallyindicated at 1 and is shown connected in a typical installation to ananode 2 buried adjacent but out of contact with a metal gasoline tank 3or the like subject to corrosion. Current for the protective system 1may be supplied from an external 117 volt A.C. source connected to thelines 4 and 5. The line 4 leads to the tank 3 which is the cathode. Theline 5, on the other hand, is connected to the anode 2 through asiliconcontrolled rectifier 8 which when triggered by current suppliedto its control gate 9 produces a pulsating halfwave unfiltered directcurrent. However, until a current signal of the correct magnitude isprovided at the control gate 9, the SCR 8 will not conduct at all.

For supplying the gate signal, the protective system 1 incorporates aphase shifting network 10 which includes two resistors 11 and 12 and acapacitor 13 connected in series across the SCR 8, and a unijunctiontransistor 14 whose base B2 and emitter E are connected across theresistor 12 and whose base B1 is connected directly to the control gate9 of the SCR.

The resistor 11 is merely a bias or dropping resistor for maintainingthe peak voltage which is applied to the unijunction transistor 14within a safe level for the transistor, In this particular example, theresistor 11 is a 20K ohm, 1 watt, 5 percent tolerance resistor.

The capacitor 13 is charged through the resistor 12 until the emitter Evoltage reaches the peak point voltage, which is the point at which thevoltage across the capacitor 13 rises above the voltage between theemitter E and the base B2 of the transistor 14, causing the transistor14 to fire and trigger the SCR 8 through the base B1 and gate 9. Thus,the capacitor 13 causes a DC. potential to be established between thebase B2 and bas B1 of the transistor 14. When the transistor 14 isfired, the capacitor 13 is then discharged through the output circuit atthe anode 2.

The function of the resistor 12 is to determine the amount of currentoutput of the transistor 14 at the base B1 which is supplied to the gateof the SCR 8. By varying the amount of resistance of the resistor 12,the amount of current flow through the unijunction transistor 14 may bevaried, the higher the resistance of the resistor 12, the more currentflowing through the unijunction transistor with consequent highervoltage through the SCR, and vice versa.

Once the SCR 8 is conducting, however, it will supply a voltage of asubstantially constant magnitude at the anode 2, and will continue tosupply such constant voltage until the current flow through the SCR isinterrupted. Thus, as soon as the SCR 8 has been triggered, the gate 9has no further elfect on the SCR and cannot turn the device off.However, when the SCR circuit is opened, the SCR 8 will no longerprovide a DC. output. This is particularly advantageous Where thestructure being protected contains a flammable product such as gasoline,since if the lines of the system are accidentally cut, the SCR 8 willimmediately cause the system to go off, thus eliminating any possibilityof sparking around the gasoline tank 3. V

The protective system 1 of the present invention has other advantages aswell. Due to the construction of the SCR 8, for example, it requiresmuch less driving power than even a typical power transistor, whichpermits the system to operate at a very low temperature for longercomponent life. Moreover, with this system any desired low voltageoutput of a substantially constant magnitude may be produced at theanode 2, it only being necessary to change the value of the resistor 12to achieve the desired voltage output, the higher the resistance of theresistor 12, the higher the voltage output and vice versa.

For leveling out the pulsating direct current output provided by the SCR8, a choke coil 20 may be inserted in the line 5 in series between theSCR 8 and anode 2. As is Well known, a choke coil will regulate currentto a limited extent due to its tendency to prevent curren from buildingup or dying down.

As fully discussed inthe aforementioned Husock patent, even though moststructures require more than 100 milliamperes for protection, it ispreferred that the output from each anode 2 be of this approximatemagnitude to guard against any undesired exposure of neighboringstructures. Accordingly, a plurality of anodes 2 with associatedcomponents must be used for a particular application to provide theamount of current needed for protection.

A 100 milliamperes DC. output at 100 ohms load re sistance may beprovided with this system by using a 20K ohm, 1 watt, 5 percenttolerance resistor 11; a 2.7K ohm, 1 watt, 5 percent tolerance resistor12; and a 0.1 microfarad, 200 w.v. D.C., percent tolerance capacitor 13.The choke may be a 1.5 henry, 200 milliampere D0,, 75 ohm coil. Inactual practice, it has been found that this particular system willproduce an output of from 0 to 134 milliamperes over a 0 to 400 ohm loadrange.

From the above discussion, it can now be seen that the protective systemof the present invention is of a unique construction, capable ofproducing a substantially uniform voltage output of any desired lowmagnitude with good current regulation while creating almost no heat forlonger component life. Moreover, should a break occur anywhere in thesystem, it will automatically shut off, thus completely eliminating anydanger of sparking in the vicinity of the structure being protected.

Other modes of applying the principles of the inven tion may beemployed, change being made as regards the details described, providedthe features stated in any of the following claims or the equivalent ofsuch be employed.

I therefore, particularly point out and distinctly claim as myinvention:

1. A protective system for objects subject to corrosion comprising atleast one anode to be submerged in an electrolyte in which such objectis located closely adjacent but out of contact with such object, asource of alternating current power, a lead for carrying electriccurrent connected to said anode, a choke coil connected in series withsaid lead, a silicon-controlled rectifier connected in series with saidchoke coil and said power source, a pair of resistors and a capacitorconnected in series across said silicon-controlled rectifier, aunijunction transistor, said unijunction transistor comprising a pair ofbase elements and an emitter, one of said base elements and said emitterbeing connected across one of said resistors which is nearest saidcapacitor, said other base element being connected directly to thecontrol gate of said silicon-controlled rectifier, and a second leadadapted to be connected from said power source to such object as thecathode.

2. The protective system of claim 1 wherein there are a plurality ofsaid anodes with associated silicon-controlled rectifiers, resistors,capacitors, choke coils and transistors connected thereto as aforesaid.

3. A protective system for underground metal storage tanks and the likecomprising a source of alternating current power, an anode adapted to beburied adjacent but out of contact with such tank, a choke coilconnected at one end to said anode, a silicon-controlled rectifierhaving anode, cathode and gate electrodes, said cathode electrodeconnected to the other end of said choke coil and said anode electrodeconnected to said power source, a unijunction transistor for triggeringsaid silicon-controlled rectifier, said transistor having base one, basetwo and emitter electrodes, the base one electrode being connecteddirectly to said gate electrode for supplying triggering current to saidsilicon-controlled rectifier, a line connecting said power source andsuch tank for completing an electrical circuit, a phase-shift network inseries connection between said source of power and the other end of saidchoke coil, said network being operative only with continuity of suchcircuit for providing set-point voltage to said transistor in each cycleof the power source for triggering said silicon-controlled rectifier.

4. A protective system as set forth in claim 3 wherein said networkcomprises first and second resistors and a capacitor in seriesconnection, said first resistor being connected to said source of powerand the base two electrode of said'transistor, said second resistorbeing connected between the base two and emitter electrodes of saidtransistor, and said capacitor being connected between the emitterelectrode and said choke coil whereby a voltage sufficient to energizesaid transistor is developed across said condenser in each cycle of saidpower source when current flows through said choke coil.

References Cited UNITED STATES PATENTS 3,162,772 12/1964 Smith 3072523,277,362 10/1966 Elliott 307-301 3,374,162 3/1968 Rubelmann 204-196HOWARD S. WILLIAMS, Primary Examiner T. TUNG, Assistant Examiner US. Cl.X.R. 307

